Major cellular system types include those operating according to the Global Services for Mobile (GSM) Standard, the TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual Mode Wide Band Spread Spectrum Cellular System (IS-95A), the TIA/EIA/IS-136 Mobile Station-Base Station Compatibility Standard (IS-136), and the TIA/EIA 553 Analog Standard (AMPS/TACS). Other major cellular systems include those operating in the personal communications system (PCS) band according to the IS-95 based ANSI-J-STD-008 1.8-2.0 GHz standard or those operating according to the GSM based PSC1900 (1900 MHz frequency range) standard. IS-95A is currently being updated, as IS-95B, in the document TIA/EIA SP-3693.
Currently, each of the major cellular system standards bodies is implementing data services into its digital cellular specifications. A packet data service specification has been finalized for GSM and IS-95A. Packet data service specifications compatible with the IS-136 and IS-95B standards are also being prepared.
In IS-95A the packet data service (IS-707) is implemented on a single channel, which is used on a make and break basis for multiple connections over the duration of a packet data session. Once a channel is seized, it is held by a call until it is timed out. The IS-707 scheme is inefficient and does not allow efficient utilization of channels by multiple packet data users. In the IS-95B packet data service (IS-707A), a fundamental channel is assigned and held similarly to the IS-95A fundamental channel, but a packet data call may also be assigned one or more supplemental channels, as necessary and as available, by a call control function. The supplemental channel packet function is handled by the Medium Access Control (MAC) function of the base station (BS). The packet data service is similar to true contention-based MAC principles as far as the handling of the supplemental channels. Supplemental channels are allocated among various users by the MAC function according to the relative fill levels of buffers set up for each call. Because the fundamental channel provides chip level synchronization information for the supplemental channels, physical layer access delays to supplemental channels may actually be reduced as compared to contention-based access schemes. The BS monitors all packet users buffers regularly and allows the supplemental channels to be shared by the users in a round-robin fashion. On the reverse link, the BS monitors requests from all the mobiles using the packet data service and allocates or de-allocates the supplemental channels accordingly.
A problem with IS-707 and IS-707A-based packet data services lies in the inefficiency caused by use of the fundamental channel. Assigning a fundamental channel to each packet call automatically takes 9.6 kbps of resources from the total resources available. Each user assigned a fundamental channel requires dedicated BS hardwire resources until the fundamental channel is timed out. If the time-out period is shortened, this may cause a load in terms of network signaling and circuit switching. Overall, this scheme may lead to excessive packet delay due to call setup delays or excessive blocking as load increases and fundamental channels become the dominant portion of total resource consumption. At the point where excessive blocking occurs, a true contention-based system would allow more effective sharing of limited resources.